Injection systems and in particular oil leakage free common rail injection systems require a control element such as for example a piezoactuator or a piezoceramic multilayer actuator in the high pressure area. In order to ensure the piezoactuator's capacity to work even under high pressures exceeding 2000 bar, the pressure must also be able to act in a lateral manner on the piezo-stack or the piezoceramic body in order to support the expansion capacity of the piezo-stack of the piezoactuator.
Such a piezoactuator is for example described in WO 02/061856 A1. In this patent, the ceramic body of this piezoactuator is enveloped in a polymer sleeve or a plastic sleeve. However, hermetic sealing of the ceramic body against fuel under a high fuel pressure such as for example 2000 bar in the case of the plastics known up to the application date of this patent application is hardly feasible or not feasible at all. Because of an electrical conductivity of commercial fuels in some cases, for example due to a slight acid content, even a slight wetting of the piezoceramics can result in voltage flashovers between the inner electrodes of the piezoactuator. In addition to this, high expansions of the plastic sleeve occur at the polarity cracks, which aggravate this problem. In addition, in WO 02/061856 the use of a filler material between the piezo-stack and the polymer sleeve or plastic sleeve is described. However, in the case of the described filler material, the problem exists that, in the case of an expansion of the piezo-stack, it may flow into developing spaces or joints, and can be destroyed in the case of a movement of the piezo-stack in the opposite direction. In this way, the filler material is consumed or destroyed over the service life of the piezo actuator. However, the reduced filler material naturally determines that the pressure from the outside on the piezoactuator can no longer be transmitted efficiently to the piezo-stack.
In addition, a solution for the transmission of the pressure to the piezo-stack of the piezoactuator with a hermetically sealed metal sheath is known, which transmits the hydraulic forces by means of a filler material, for example of plastic, to the side surfaces of the piezoceramic body. However, this solution well known to the applicant has the disadvantage that the thermal expansion coefficients of plastics exceed those of the piezoceramics by orders of magnitude, which consequently require volume equalization for example by means of air or gas-filled cavities. However, these required cavities considerably reduce the laterally transmittable forces to the piezoceramics or the piezo-stack so that support for the longitudinal expansion of the piezo-stack is significantly reduced.
In addition, in this known solution there exists the problem of the increased space requirement need for the actuator. Because of the high thermal expansion coefficients of plastics compared with the metal sheath, the filler material may not exceed a certain volume since otherwise the maximum yieldable stress amplitudes of the metal sheath would be exceeded with the extreme combinations arising from temperature and pressure during the operation. Where the cavities between the sheath and the piezo-stack are filled as described above, the necessary volume minimization for a compact and space-saving construction of the multilayer-actuator is difficult or impossible because a comparatively large distance must be selected between the inner diameter of the metal sheath and the outer contacting of the piezo-stack in order to guarantee a sufficient passivation thickness everywhere in the case of the given production-related shape inaccuracies as well as the operation-relation deformations of the piezo-stack and the metal sheath.
Furthermore, contacting of the piezo-stack with a compact design is known, which consists of a metal pin of approximately 1 mm diameter for the respective plus and the minus poles, which in each case is connected electrically conductively by means of about 60 wires to the outer metallization of the piezo-stack. These pins preferably extend over the entire length of the piezo-stack. In this process, the wires are fixed equally spaced to the pin and the outer metallization of the piezo-stack so that with an interruption in the metallization or a wire, a maximum of 1/60 of the electrical capacity of the piezo-stack is lost. However, this solution is not suitable to the effect that the contacting, in particular the pins for the movements of the plus and the minus poles take part in particular expansions of the piezo-stack. In addition, bringing about contact by means of metal pins and associated wires for contacting the metallization of the piezo-stack is expensive. Furthermore, the current carrying capacity of the outer contacting or the outer electrode of the piezo-stack consisting of pins and wires is low. This, in particular in the case of local short circuits, leads to problems which can lead to the destruction of the multilayer actuator.